To claim Input Tax Credits for your home office expenses when you're filing your GST HST returns, your work space needs to be, in the Canada Revenue Agency's words, "your principal place of business; or used 90% or more to earn income from your business and used on a regular and continuous basis for meeting your clients, customers, or patients".

(I just made up the bit about the inspection program. Really!)

The same basic rule of use applies to income tax deductions for home based businesses. 6 Home Based Business Deductions You Don't Want to Miss provides more information on the kinds of tax deductions home based business owners can make on their income tax.